Vented extrusion is a common means for removing volatiles from a polymer during processing. Typical volatiles include, but are not limited to, oligomers, residual solvents, and moisture. Conventional vented extruders have a vent located about halfway between the feed hopper and the die (i.e. about 10 screw diameters from the hopper where the total screw length is typically 24 to 30 diameters) so that venting is performed after the polymer has melted. The vent is typically located on the surface of the barrel, i.e. a vent port, with direct exposure to the atmosphere. Another venting option is to remove the volatiles through a screw having a hollow core connecting with lateral holes in the extraction zone.
In FIG. 1, a conventional vented single screw extruder is shown. The vent is typically about ten diameters from the die end of the extruder. Polymer is fed from the hopper, through the feed section, and into the transition section. The transition section is denoted by the increasing root diameter of the screw. In the feed section, the channel is only partially filled over a short distance, and over this region, no pressure generation occurs. The actual degree of filling depends on the shape of the material fed (i.e. pellets, powders) and the method of feeding. Normally, the screw is fed from a full hopper as depicted in FIG. 1 which maximizes the degree of fill. However, the length of the partially filled section can be increased by "starve-feeding" the extruder via some type of metering device. In the transition section, the screw with its increasing diameter acts to compress the pellets and squeeze out air and free space. This action generates pressure. By the end of the first transition section, the polymer is completely melted and there is no unoccupied space in the barrel.
In order to vent the polymer to the atmosphere or create a vacuum, the melt pressure must be reduced to essentially zero. One way to accomplish this is by decompression wherein the screw root diameter is decreased so that the channel is again only partially filled. In this partially filled area, the pressure is low and venting can occur without polymer squirting out of the barrel. Volatiles can be stripped from the exposed surfaces of the polymer as the polymer transitions through the vent area into the second transition section. Once the polymer reaches the second transition section, the polymer recompresses, the channel again becomes filled, and the polymer is pumped to the metering section.
However, since venting follows melting, the polymer experiences sufficient time and high temperatures with moisture in the melt that some hydrolysis occurs before the polymer reaches the vent area. The hydrolysis results in loss of molecular weight, surface imperfections, and poor overall properties of the polymer. The conventional vented extruder systems are thus incapable of removing sufficient levels of moisture in the polymer before hydrolytic degradation occurs. In particular, the removal of moisture is important for polyesters. Since conventional venting techniques are incapable of removing moisture, polyesters often undergo a drying process prior to extrusion. Drying is often cost prohibitive because additional equipment, space, and energy is required
In twin screw extrusion, multiple vents are commonly utilized including a rear vent located behind the feed hopper. Generally these rear vents are only considered effective for large-scale devolatilization when the feed material is liquid, slurry, or melt. The rear vents function similarly and are just as effective as standard vents in twin screws when the feed material is in a liquid form.
Rear venting has been used in single screw extruders, but only in cases where the feed material is already molten. Such is the case when a compounding extruder is fed into a devolatilization extruder or when the feed is a polymer-solvent solution. For many applications, the volatile involved does not degrade the polymer so early extraction is not needed. Also, the volatile is typically distributed evenly throughout the pellet, thus feeding a polymer melt versus cold pellets would result in only a minor change of devolatilization efficiency.
For polystyrene and polyolefins processes, the feed material is often made in a suspension or solution form. The solvent must be removed in order to make solid pellets that can be easily handled and shipped. Since degradation is not an issue, rapid removal of the solvent is not a concern. These resins can also contain toxic monomers, such as styrene in polystyrene or bis-A in polycarbonate, that must be removed. Usually these suspension polymers are fed in this slurry form right into the mouth of a twin screw or some other type of devolatilizer, which is why rear venting works well.
Thus, there exists a need in the art for a low capital, cost advantage method for rapid and early removal of moisture from a polymer susceptible to hydrolytic degradation to minimize hydrolysis. Accordingly, it is to the provision of such method that the present invention is primarily directed.